Hot-rolled steel sheet and method of manufacturing same

ABSTRACT

This hot-rolled steel sheet has a predetermined chemical composition, in which a microstructure contains, by area %, bainite: 80.0% or more, ferrite: 10.0% or less, and a remainder in the microstructure: 10.0% or less, a total density of a length L 7  of a grain boundary having a crystal orientation difference of 7° and a length L 68  of a grain boundary having a crystal orientation difference of 68° about a &lt;110&gt; direction in the bainite is 0.35 to 0.60 μm/μm 2 , and a tensile strength is 780 MPa or more.

TECHNICAL FIELD OF THE INVENTION

The present invention relates to a hot-rolled steel sheet and a methodof manufacturing the same. Specifically, the present, invention relatesto a hot-rolled steel sheet having high strength and excellentductility, hole expansibility, and toughness, and a method ofmanufacturing the same.

Priority is claimed on Japanese Patent Application No. 2019-201427,filed on Nov. 6, 2019, the content of which is incorporated herein byreference.

BACKGROUND ART

In recent years, from the viewpoint of protecting the globalenvironment, efforts, have been made to reduce the amount of carbondioxide gas emitted in many fields. Vehicle manufacturers are alsoactively developing techniques for reducing the weight of vehicle bodiesfor the purpose of reducing fuel consumption. However, it is not easy toreduce the weight of vehicle bodies since the emphasis is placed onimprovement in collision properties to secure the safety of theoccupants.

In order to achieve both vehicle body weight reduction and collisionproperties, an investigation has been conducted to make a member thin byusing a high strength steel sheet. Therefore, a steel sheet having bothhigh strength and excellent formability is strongly desired. A steelsheet having excellent ductility and hole expansibility particularlyamong the formability is desired. In addition, a steel sheet applied toa vehicle body of a vehicle is also required to have excellent toughnessin order to sufficiently absorb impact at the time of a collision.

For example, Patent Document 1 discloses a high strength hot-rolledsteel sheet which has excellent fatigue properties and stretchflangeability and in which when a bainite fraction is 80% or more, anaverage grain size r (nm) of a precipitation satisfies an expression of(r≥207/(27.4×(V)+23.5×(Nb)+31.4×(Ti)+17.6×(Mo)+25.5×(Zr)+23.5×(W)), andthe average grain size rand a precipitation fraction f satisfies anexpression of (r/f≤12000).

Patent Document 2 discloses a hot-rolled steel sheet in which a steelstructure at a position at a depth of ¼ of a sheet thickness from asurface of the steel sheet contains, by area %, 60% or more, of bainite,5% or more and less than 30% of polygonal ferrite, less than 3% ofresidual austenite, and 10% or less of a remainder excluding thebainite, the residual austenite, and the polygonal ferrite, and apolygonal ferrite area ratio at, a position at a depth of 100 μm fromthe surface of the steel sheet and a polygonal ferrite area ratio at aposition at a depth of ¼ of the sheet thickness satisfy an expression of(Vαs>1.5 Vαq, where Vαs is an area ratio (%) of the polygonal ferrite aa position at a depth of 100 μm from the surface of the steel sheet, andVαq is an area ratio of the polygonal ferrite at a position of a depthof ¼ of the sheet thickness from the surface of the steel sheet).

PRIOR ART DOCUMENT Patent Document

[Patent Document 1] Japanese Unexamined Patent Application, FirstPublication No. 2009-84637

[Patent Document 2] Japanese Unexamined Patent Application, FirstPublication No. 2016-50335

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention

However, in Patent Documents 1 and 2, toughness is not considered. Thepresent inventors have found that it is necessary not only to improveductility and hole expansibility but also to secure toughness, in orderto achieve both weight reduction, of a vehicle body and collisionproperties.

The present invention has been made in view of the above problems, andan object of the present invention is to provide a hot-rolled steelsheet having high strength and excellent ductility, hole expansibility,and toughness, and a method of manufacturing the same.

In addition, a steel sheet applied to a vehicle body of a vehicle may berequired to have excellent punching properties in addition to theabove-mentioned properties in some cases. Therefore, an object of thepresent invention is, preferably, to provide a hot-rolled steel sheethaving excellent punching properties in addition to the above-mentionedproperties and a method of manufacturing the same.

Means for Solving the Problem

In view of the above-mentioned problems, as a result of intensiveinvestigations on the chemical composition of a hot-rolled steel sheetand the relationship between a microstructure and mechanical properties,the present inventors have obtained the following findings (a) to (e)and thus completed the present invention.

(a) In order to obtain excellent ductility and hole expansibility, it isnecessary to make a total area ratio of bainite 80.0% or more.

(b) By controlling a grain boundary density with a specific orientationin bainite, the ductility, the hole expansibility, and the toughness canbe further improved.

(c) In order to make the grain boundary density with the specificorientation in bainite within a desired range, it is necessary tocontrol a winding temperature and a retention temperature and retentiontime after winding.

(d) In order to improve the punching properties, it is necessary tocontrol an average grain size and an aspect ratio of prior austenitegrains.

(e) In order to obtain the desired average grain size and the aspectratio of the prior austenite grains, it is necessary to control a hotrolling condition more strictly. Specifically, in a hot rolling step, itis necessary to control a toted rolling reduction of rough rolling androlling reductions of final three stages of finish rolling.

The gist of the present invention made based on the above findings is asfollows.

(1) A hot-rolled steel sheet according to an aspect of the presentinvention includes, as a chemical composition, by mass %.

C: 0.030% to 0.200%;

Si: 0.05% to 2.50%;

Mn: 1.00% to 4.00%;

sol. Al: 0.001% to 2.000%;

Ti: 0.030% to 0.200%,

P: 0.020% or less;

S: 0.020% or less;

N: 0.010% or less;

Nb: 0% to 0.200%;

B: 0% to 0.010%;

V: 0% to 1.00%;

Mo: 0% to 1.00%;

Cu: 0% to 1.00%;

W: 0% to 1.00%;

Cr: 0% to 1.00%;

Ni: 0% to 1.00%;

Co: 0% to 1.00%;

Ca: 0% to 0.010%;

Mg: 0% to 0.010%;

REM: 0% to 0.010%;

Zr: 0% to 0.010%; and

a remainder consisting of iron and impurities,

in which a microstructure contains, by area %,

bainite: 80.0% or more,

ferrite: 10.0% or less, and

a remainder in the microstructure: 10.0% or less,

a total density of a length L₇ of a grain boundary having a crystalorientation difference of 7° and a length L₆₈ of a grain boundary havinga crystal orientation difference of 68° about a <110> direction in thebainite is 0.35 to 0.60 μm/μm², and

a tensile strength is 780 MPa or more.

(2) The hot-rolled steel sheet according to (1) may include, as achemical composition, by mass %, one or two or more selected from thegroup consisting of:

Nb: 0.005% to 0.200%;

B: 0.001% to 0.010%;

V: 0.005% to 1.00%;

Mo: 0.005% to 1.00%;

Cu: 0.005% to 1.00%;

W: 0.005% to 1.00%;

Cr: 0.005% to 1.00%;

Ni: 0.005% to 1.00%;

Co: 0.005% to 1.00%;

Ca: 0.0005% to 0.010%;

Mg: 0.0005% to 0.010%;

REM: 0.0005% to 0.010%; and

Zr: 0.0005% to 0.010%.

(3) The hot-rolled steel sheet according to (1) or (2), in which in themicrostructure, an average grain size of prior austenite grains is 10 to30 μm, and a ratio I_(d)/S_(d) between a long axis I_(d) and a shortaxis S_(d) of the prior austenite grains may be 2.0 or less.

(4) A method of manufacturing a hot-rolled steel, sheet according toanother aspect of the present invention includes:

a heating step of retaining a slab having the chemical compositionaccording to (1) above at a heating temperature of 1200° C. or higherfor 1.0 hour or longer;

a hot rolling step of performing rough rolling so that a rough rollingcompletion temperature is 1000° C. or higher and a total rollingreduction is more than 65%, and performing finish rolling so that afinish rolling completion temperature is 860° C. to 980° C.; and

a cooling step of performing cooling to a temperature range of 570° C.to 620° C. at an average cooling rate of 20° C./s or higher andperforming winding, then, performing retaining at a temperature range of500° C. to 580° C. for 2.0 to 12.0 hours, and then performing cooling toa room temperature.

(5) The method of manufacturing a hot-rolled steel sheet according to(4) above, in which in the hot rolling step, the total rolling reductionin the rough rolling is set to 70% or more, and the finish rolling maybe performed so that all rolling reductions of final three stages of thefinish rolling are less than 25%.

Effects of the Invention

According to the above aspect according to the present invention, it ispossible to provide a hot-rolled steel sheet having high strength, andexcellent ductility, hole expansibility, and toughness, and a method ofmanufacturing the same. According to the above preferred aspectaccording to the present invention, it is possible to provide ahot-rolled steel sheet having excellent punching properties in additionto the above-mentioned properties and a method of manufacturing thesame.

EMBODIMENTS OF THE INVENTION

The chemical composition and the microstructure of a hot-rolled steelsheet (hereinafter, sometimes simply referred to as a steel sheet)according to the present embodiment will be described in detail below.However, the present invention is not limited to the, configurationdisclosed in the present embodiment, and various modifications can bemade without departing from the scope of the present invention.

The numerical limit range described with “to” in between includes thelower limit and the upper limit. Regarding the numerical value indicatedby “less than” or “more than”, the value, does not fall within thenumerical range. In the following description, % regarding the chemicalcomposition is mass % unless otherwise specified.

Chemical Composition

The-hot-rolled steel sheet according to the present embodiment includes,by mass % C: 0.030% to 0.200%, Si: 0.05% to 2.50%, Mn: 1.00% to 4.00%,sol. Al: 0.001% to 2.000%, Ti: 0.030% to 0.200, P: 0.020% or less, S:0.020% or less, N: 0.010% or less, and a remainder: Fe and impurities.Each element will be described in detail below.

C: 0.030% to 0.200%

C is an element that promotes formation of bainite by improving astrength of a hot-rolled steel sheet and also improving hardenability.In order to obtain this effect, a C content is set to 0.030% or more.The C content is preferably 0.040% or more.

On the other hand, when the C content is more than 0.200%, it becomesdifficult to control the formation of bainite, a large amount ofmartensite is formed, and one or both of ductility and holeexpansibility of the hot-rolled steel sheet is decreased. Therefore, theC content is set to 0.200% or less. The C content is preferably 0.180%or less.

Si: 0.05% to 2.50%

Si is an element that contributes to solid solution strengthening and isan element that contributes to improving the strength of the hot-rolledsteel sheet. In addition Si has an action of making steel soundness bydeoxidation (suppressing an occurrence of a defect such as blow holes inthe steel). When a Si content is less than 0.05%, an effect by theaction cannot be obtained. Therefore, the Si content is set to 0.05% ormore. The Si content is preferably 0.50% or more and more preferably1.00% or more.

On the other hand, Si is an element that promotes formation of a mixture(MA) of full hard martensite (hereinafter, when simply referred to asmartensite, this martensite means fresh martensite) and residualaustenite. When the Si content is more than 2.50%, MA is formed and thehole expansibility of the hot-rolled steel sheet is decreased.Therefore, the Si content is set to 2.50% or less. The Si content ispreferably 2.30% or less and more preferably 2.00% or less.

Mn: 1.00% to 4.00%

Mn dissolves in steel to contribute to an increase in the strength ofthe hot-rolled steel sheet, promotes the formation of bainite byimproving the hardenability and improves the hole, expansibility of thehot-rolled steel sheet. In order to obtain such an effect, a Mn contentis set to 1.00% or more. The Mn content is preferably 1.30% or more.

On the other hand, when the Mn content is more than 4.00%, it becomesdifficult to control the formation, of the bainite, a desired amount ofbainite cannot be obtained, and one or both of the ductility and thehole expansibility of the hot-rolled steel sheet is decreased.Therefore, the Mn content is set to 4.00% or less. The Mn content ispreferably 3.50% or less.

sol. Al: 0.001% to 2.000%

Similar to Si, Al has an action of deoxidizing steel to make the steelsoundness. When a sol. Al content is less than 0.001%, an effect by theaction cannot be obtained. Therefore, the sol. Al content is set to0.001% or more. The sol. Al content is preferably 0.010% or more.

On the other hand, when the sol. Al content is more than 2.000%, anincrease of an oxide-based inclusion is caused, and the holeexpansibility of the hot-rolled steel sheet is decreased. Therefore, thesol. Al content is set to 2.000% or less. The sol. Al content ispreferably 1,500% or less and more preferably 1.300% or less.

The sol. Al in the present embodiment means acid-soluble Al, and refersto solid solution Al present in steel in a solid solution state.

Ti: 0.030% to 0.200%

Ti precipitates as a carbide or a nitride in steel, and has an action ofrefining the microstructure by an austenite pinning effect and improvingthe strength of the hot-rolled steel sheet. When a Ti content is lessthan 0.030%, an effect by the action cannot be obtained. Therefore, theTi content is set to 0.030% or more. The Ti content is preferably 0.050%or more and more preferably 0.080% or more.

On the other hand, when the Ti content is more than 0.200%, the prioraustenite grains are less likely to recrystallize, and a rolled texturedevelops, resulting in decrease in the hole expansibility of thehot-rolled steel sheet. Therefore, the Ti content is set to 0.200% orless. The Ti content is preferably 0.170% or less, and more preferably0.150% or less.

P: 0.020% or less

P is an element that dissolves in steel and contributes to an increaseof the strength of the hot-rolled steel sheet. However, P is also anelement that segregates at a grain boundary, particularly at a prioraustenite grain boundary, and promotes a grain boundary fracture, due toa boundary segregation, thereby causing a decrease in the workability ofthe hot-rolled steel sheet. A P content is preferably as low aspossible, and containing of P is acceptable up to 0.020%. Therefore, theP content is set to 0.020% or less. The P content is preferably 0.015%or less.

The P content is preferably set to 0%. However, when the P content isreduced to less than 0.0001%, the manufacturing costs increase.Therefore, the P content may be 0.0001% or more.

S: 0.020% or less

S is an element that adversely affects weldability and manufacturabilityduring casting and hot rolling. S combines with Mn to form coarse MnS.This MnS deteriorates the bendability and hole expansibility of thehot-rolled steel sheet, and promotes an initiation of a delayedfracture. A S content is preferably as low as possible, and containingof S is acceptable up to 0.020%. Therefore, the S content is set to0.020% or less. The S content is preferably 0.015% or less.

The S content is preferably set to 0%. However, when the S content isreduced to less than 0.0001%, the manufacturing cost increases and it iseconomically disadvantageous. Therefore, the S content may be set to0.0001% or more.

N: 0.010% or less

N is an element that forms a coarse nitride in steel. This nitridedeteriorates the bendability and the hole expansibility of thehot-rolled steel sheet. Therefore, a N content is set to 0.010% or less.The N content is preferably 0.008% or less.

When the N content is reduced to less than 0.0001%, a significantincrease in manufacturing cost is caused. Therefore, the N content maybe set to 0.0001% or more.

A remainder of the chemical composition of the hot-rolled steel sheetaccording to the present embodiment consists of Fe and impurities. Inthe present embodiment, the impurities mean those mixed from ore as araw material, scrap, manufacturing environment, and the like, and/orthose allowed within a range that does not adversely affect thehot-rolled steel sheet according to the present embodiment.

The hot-rolled steel sheet according to the present embodiment maycontain the following elements as an optional element in addition to apart of Fe. In a case where the above optional element is not contained,a lower limit of a content thereof is 0%. Hereinafter, each optionalelement will be described in detail.

Nb: 0% to 0.200%

Nb is an element that forms a carbide during hot rolling and contributesto improvement in the strength of hot-rolled steel sheet byprecipitation hardening. In order to reliably obtain the effect, a Nbcontent is preferably set to 0.005% or more.

On the other hand, when the Nb content is more than 0.200%, arecrystallization temperature of the prior austenite grains becomes toohigh and a texture develops, and the hole expansibility of thehot-rolled steel sheet may be decreased in some cases. Therefore, the Nbcontent is set to 0.200% or less.

B: 0% to 0.010%

B is an element that segregates into the prior austenite grain boundary,suppresses the formation and growth of ferrite, and contributes toimprovement in the strength and hole expansibility of the hot-rolledsteel sheet. In order to reliably obtain these effects, a B content ispreferably set to 0.001% or more.

On the other hand, even when B is contained in an amount more than0,010%, the above effects are saturated. Therefore, the B content is setto 0.010% or less.

V: 0% to 1.00%

V is an element that forms a carbonitride during hot rolling andcontributes to improvement in the strength of hot-rolled steel sheet byprecipitation hardening. In order to reliably obtain the effect, a Vcontent is preferably set to 0.005% or more.

On the other hand, when the V content is more than 1.00%, a coarsecarbide is formed in the slab, which causes an initiation of cracking inthe heating step. Therefore, the V content is set to 1.00% or less.

Mo: 0% to 1.00%

Mo is an element, that promotes the formation of bainite by improvingthe hardenability of steel and contributes to the improvement in thestrength and the hole expansibility of the hot-rolled steel sheet. Inorder to reliably obtain the effect, a Mo content is preferably set to0.005% or more.

On the other hand, when the Mo content is more than 1.00%, martensite islikely to be formed, and one or both of elongation and the holeexpansibility of the hot-rolled steel sheet may be decreased in somecases. Therefore, the Mo content is set to 1.00% or less.

Cu: 0% to 1.00%

Cu is an element that has an effect for stably securing the strength ofthe hot-rolled steel sheet. Therefore, Cu may also be contained.However, even when containing Cu in an amount more than 1.00%, theeffect of the action is likely to be saturated and may be economicallydisadvantageous in some cases. Therefore, the Cu content is set to 1.00%or less. The Cu content is preferably 0.80% or less and more preferably0.50% or less. In order to more reliably obtain the effect by theaction, the Cu content is preferably 0.005% or more.

W: 0% to 1.00%

W is an element that is effective in improving the strength of thehot-rolled steel sheet by solid or precipitation. However, even whencontaining W in an amount more than 1.00%, the effect of the action islikely to be saturated and may be economically disadvantageous in somecases. Therefore, a W content is set to 1.00% or less. The W content ispreferably 0.80% or less and more preferably 0.50% or less. In order tomore reliably obtain the effect by the action, the W content ispreferably 0.005% or more.

Cr: 0% to 1.00%

Cr is an element that is effective in improving the hardenability andimproving the strength of the hot-rolled steel sheet. However, even whencontaining Cr in an amount more than 1.00%, the effect of the action islikely to be saturated and may be economically disadvantageous in somecases. Accordingly, a Cr content is set to 1.00% or less. The Cr contentis preferably 0.80% or less and more preferably 0.50% or less. In orderto more reliably obtain the effect by the action, the Cr content ispreferably 0.005% or more.

Ni: 0% to 1.00%

Ni is an element that is effective in improving the hardenability andimproving the strength of the hot-rolled steel shed. However, whencontaining Ni in an amount more than 1.00%, the hardenability isexcessively increased and a microstructural fraction of martensite isincreased, so that the hole expansibility of the hot-rolled steel sheetmay deteriorate in some cases. Therefore, a Ni content is set to 1.00%or less. The Ni content is preferably 0.80% or less and more preferably0.50% or less. In order to more reliably obtain the effect by theaction, the Ni content is preferably 0.005% or more.

Co: 0% to 1.00%

Co is an element that is effective in improving the strength of thehot-rolled steel sheet by solid solution strengthening. However, evenwhen containing Co in an amount more than 1.00%, the effect of theaction is likely to be saturated and may be economically disadvantageousin some cases. Accordingly, a Co content is set to 1.00% or less. The Cocontent is preferably 0.80% or less and more preferably 0.50% or less.In order to more reliably obtain the effect by the action, the Cocontent is preferably 0.005% or more.

Ca: 0% to 0.010%

Mg: 0% to 0.010%

REM: 0% to 0.010%

Zr: 0% to 0.010%

All of calcium (Ca), magnesium (Mg), a rare earth element (REM), andzirconium (Zr) are elements that contribute to inclusion control,especially fine dispersion of an inclusion, and has an action ofenhancing the toughness of the hot-rolled steel sheet. Therefore, theseelements may be contained. However, when each of the elements iscontained in an amount of more than 0.010%, deterioration of surfaceproperties may become apparent in some cases. Therefore, the amount ofeach of these elements is set to 0.010% or less. Each amount of theseelements is preferably 0.005% or less and, more preferably 0.003% orless. In order to obtain the effect by the action more reliably, eachamount of the elements is preferably 0.0005% or more.

REM in the present embodiment refers to a total of 17 elements including, Y, and lanthanoid, and the REM content refers to a total amount ofthese elements. In a case of the lanthanoid, lanthanoid is industriallyadded in the form of misch metal.

The chemical composition of the hot-rolled steel sheet may be measuredby a general analytical method. For example, the chemical compositionmay be measured using inductively coupled plasma-atomic emissionspectroscopy (ICP-AES) or optical emission spectroscopic (OES). Notedthat, C and S may be measured by using a combustion-infrared absorptionmethod and N may be measured by using the inert, gas melting-thermalconductivity method.

Microstructure of Hot-Rolled Steel Sheet

Next, the microstructure of the hot-rolled steel sheet according to thepresent embodiment will be described.

In the hot-rolled steel sheet according to the present embodiment, amicrostructure contains, by area %, bainite: 80.0% or more, ferrite:10.0% or less, and a remainder in the microstructure: 10.0% or less, anda total density of a length L₇ of a grain boundary having a crystalorientation difference of 7° and a length L₆₈ of a grain boundary havinga crystal orientation difference of 68° about a <110> direction in thebainite is 0.35 to 0.60 μm/μm².

In addition, in the hot-rolled steel sheet according to the, presentembodiment, in the microstructure, an average grain size of prioraustenite grains is 10 to 30 μm, and a ratio I_(d)/S_(d) between a longaxis I_(d) and a short axis S_(d) of the prior austenite grains may be2.0 or less.

In the present embodiment, the microstructure is defined at a depth of ¼of the sheet thickness from a surface and a center position in a sheetwidth direction in a cross section parallel to a rolling direction. Thereason is that the microstructure, at this position is a typicalmicrostructure of a steel sheet.

Bainite: 80.0%© or more

Bainite means a lath-shaped bainitic ferrite and a structure having aFe-based carbide between and/or inside the bainitic ferrite. Unlikepolygonal ferrite, the bainitic ferrite has a lath shape and has arelatively high dislocation density inside, and therefore can be easilydistinguished from other structures using SEM or TEM.

When an area ratio of the bainite is less than 80.0%, the toughness andthe hole expansibility of the hot-rolled steel sheet are significantlydecreased. Therefore, the area ratio of the bainite is set to 80.0% ormore. The area ratio of the bainite is preferably 85.0% or more and morepreferably 90.0% or more. The higher the area ratio of the bainite, themore preferable. However, since it is difficult to achieve an area ratioof 97.5% or more due to the presence of ferrite, cementite, or MA(mixture of residual austenite and martensite), a practical upper limitmay be 97.5%.

Ferrite: 10.0% or less

Ferrite is polygonal ferrite, and the bainitic ferrite is not includedin ferrite. When an area ratio of the ferrite is more than 10.0%, adesired tensile strength cannot be obtained. Therefore, the area ratioof the ferrite is set to 10.0% or less. The area ratio of the ferrite ispreferably 5.0% or less. From the viewpoint of securing ductility, thearea ratio of the ferrite may be 1.0% or more.

Remainder in Microstructure (Cementite, Pearlite, Martensite, Temperedmartensite, and residual austenite): 10.0% or Less in Total

All of Cementite, pearlite, martensite, tempered martensite, andresidual austenite are starting points of voids during distortion, andare structures that deteriorate the hole expansibility of the hot-rolledsteel sheet. When a total area ratio of these remainder in themicrostructure is more than 10.0% the desired ductility and the holeexpansibility cannot be obtained. Therefore, the area ratio of theremainder in the microstructure (cementite, pearlite, martensite,tempered martensite, and residual austenite) is set to 10.0% or less.The area ratio thereof is preferably 5.0% or less.

On the other hand, in a microstructure control, since it is practicallydifficult to control the area ratio of the remainder in themicrostructure to less than 1.0%, the area ratio of the remainder in themicrostructure may be 1.0% or more.

In addition, the smaller the total area ratio of the martensite and thetempered martensite in the remainder in the microstructure, the morestable and excellent hole expansibility can be obtained. Therefore, thetotal area ratio of the martensite and the tempered martensite ispreferably 5.0% or less. The total area fraction thereof is morepreferably 3.0% or less.

A method of measuring an area ratio of each structure will be describedbelow.

A test piece is taken from the hot-rolled steel sheet so that amicrostructure at a depth of ¼ of the sheet thickness from the surfaceand a center position in a sheet width direction in the cross sectionparallel to the rolling direction can be observed.

After polishing the cross section of the test piece with silicon carbidepaper of #600 to #1500, finishing is performed to a mirror surface usinga diamond powder having a grain size of 1 to 6 μm using a dilutedsolution such as alcohol or a liquid dispersed in pure water. Next,polishing is performed with colloidal silica without containing analkaline solution at a room temperature to remove a strain introducedinto a surface layer of a sample.. A region with a length of 50 μm andbetween a depth of ⅛ of the sheet thickness from the surface to a depthof ⅜ of the sheet thickness from the surface is measured by electronbackscatter diffraction at a measurement interval of 0.1 μm, so that aposition at the depth of ¼ of the sheet thickness from the surface isthe center in a random position of the sample cross section in alongitudinal direction, to obtain crystal orientation information.

For the measurement, an EBSD analyzer configured of a thermal fieldemission scanning electron microscope (JSM-7001F manufactured by JEOL)and an EBSD detector (DVC5 type detector manufactured by TSL) is used.In this case, the EBSD analyzer is set such that the degree of vacuuminside is 9.6×10⁻⁵ Pa or less, an acceleration voltage is 15 kV, anirradiation current level is 13, and an electron beam irradiation levelis 62. The obtained crystal orientation information is used to calculatethe area ratio of the residual austenite using a “Phase Map” functioninstalled in the software “OIM Analysis (registered trademark)” attachedto the EBSD analyzer. Those having a crystal structure of fcc aredetermined to be residual austenite.

Next, those having a crystal structure of bcc are determined to bebainite, ferrite, and the “remainder in the microstructure (cementite,pearlite, martensite, and tempered martensite) other than residualaustenite”. In these regions, a region where the “Grain OrientationSpread” is 1° or less is extracted as ferrite, by using a “GrainOrientation Spread” function installed in the software “OIM Analysis(registered trademark)” attached to the EBSD analyzer, under thecondition, in which 15° grain boundary is defined as the grain boundary.By calculating the area ratio of the extracted ferrite, the area ratioof the ferrite is obtained.

Subsequently, under the condition in which 5° grain boundary is defined,as the grain boundary in the residual area (a region where the “GrainOrientation Spread” is more than 1°), when a maximum value of the “GrainAge IQ” of the ferrite region is set to Iα, a region exceeding Iα/2 isextracted as bainite, and a region of equal to or less than Iα/2 isextracted as “remainder in the microstructure (cementite, pearlite,martensite, and tempered martensite) other than the residual austenite”.By calculating the area ratio of the extracted bainite, the area ratioof the bainite is obtained. In addition, the area ratio of the extracted“remainder in the microstructure (cementite, pearlite, martensite, andtempered martensite) other than residual austenite” is calculated, andthe area ratio of the above residual austenite is added to obtain thearea ratio of the remainder in the microstructure (cementite, pearlite,martensite, tempered martensite, and residual austenite).

Regarding the extracted “remainder in the microstructure (cementite,pearlite, martensite, and tempered martensite) other than residualaustenite”, cementite, pearlite, martensite, and tempered martensite canbe distinguished by the following method. First, in order to observe thesame region as the EBSD measurement region by SEM, a Vickers indentationis imprinted in the vicinity of an observation position. Thereafter, acontamination on the surface layer is removed by polishing, leaving thestructure of the observed section, and nital etching is performed. Next,the same visual field as the EBSD observed section is observed by SEM ata magnification of 3000 times.

In the EBSD measurement, among the regions determined as the remainderin, the microstructure, a region having a substructure in the grain andwhere cementite precipitates with a plurality of variants is determinedto be tempered martensite. A region where the cementite precipitates ina lamellar shape is determined to be pearlite. Spherical particles withhigh brightness and grain size circle equivalent diameter) of 2 μm orless are determined to be cementite. A region where the brightness ishigh and the substructure is not exposed by etching is determined as“martensite and residual austenite”. By calculating the area ratio ofeach structure, the area ratio of the tempered martensite, the pearlite,the martensite, and the “martensite and residual austenite” is obtained.The area ratio of the martensite can be obtained by subtracting the arearatio of the residual austenite obtained by the above-mentioned EBSDfrom the area ratio of the obtained “martensite and residual austenite”.

For removing contamination on the surface layer of the, observedsection, a method such as buffing using alumina particles having aparticle diameter of 0.1 μm or less or Ar ion sputtering may be used.

Total Density of Length L₇ of Grain Boundary Having Crystal OrientationDifference of 7° and Length L₆₈ of Grain Boundary Having CrystalOrientation Difference of 68° about <110> Direction in Bainite: 0.35 to0.60 μm/μm²

When a total density of a length L₇ of a grain boundary having a crystalorientation difference of 7° and a length L₆₈ of a grain boundary havinga crystal orientation difference of 68° about a <110> direction in thebainite is set to 0.35 to 0.60 μm/μm², the ductility, the holeexpansibility, and the toughness of the hot-rolled steel sheet can beimproved.

When the, total density of the length L₇ and the length L₆₈ is less than0.35 μm/μm², the toughness of the bainite is significantly decreased,and the desired toughness cannot be obtained in the hot-rolled steelsheet. Therefore, the total density of L₇ and L₆₈ is set to 0.35 μm/μm²or more. The total density thereof is preferably 0.40 μm/μm² or more. Onthe other hand, when the total density of the length L₇ and the lengthL₆₈ is more than 0.60 μm/μm², the ductility of the bainite issignificantly decreased, and the excellent ductility and the holeexpansibility cannot be obtained in the hot-rolled steel sheet.Therefore, the total density of L₇ and L₆₈ is set to 0.60 μm/μm² orless. The total density thereof is preferably 0.55 μm/μm² or less.

The grain boundary having a crystal orientation difference of X° aboutthe <110> direction refers to a grain boundary having a crystallographicrelationship in which the crystal orientations of the crystal grain Aand, the crystal grain B are the same by rotating one crystal grain B byX° about the <110> axis, when two adjacent crystal grain A and crystalgrain B are specified at a certain grain boundary. However, consideringthe measurement accuracy of the crystal orientation, an orientationdifference of ±4° is allowed from the matching orientation relationship.

In the present embodiment, the length L₇ of a grain boundary and thelength L₆₈ as above are measured by using the electron back scatterdiffraction pattern-orientation image microscopy (EBSP-OIM) method. Inthe EBSP-OLM method, a crystal orientation of an irradiation point canbe measured for a short time period in such manner that a highlyinclined sample in a scanning electron microscope (SEM) is irradiatedwith electron beams, a Kikuchi pattern formed by back scattering isphotographed by a high sensitive camera, and the photographed image isprocessed by a computer. The EBSP-OIM method is performed using a devicein which a scanning electron microscope and an EBSP analyzer arecombined and an OIM Analysis (registered trademark) manufactured byAMETEK Inc.

In the EBSP-OIM method, since the, fine structure of the sample surfaceand the crystal orientation can be analyzed, the length of the grainboundary having a specific crystal orientation difference can bequantitatively determined. The analyzable area of the EBSP-OIM method isa region that can be observed by the SEM. The EBSP-OIM method makes itpossible to analyze a region with a minimum resolution of 20 nm, whichvaries depending on the resolution of the SEM.

When measuring the density of the length of specific grain boundary ofthe microstructure at the depth of ¼ of the sheet thickness from thesurface and at the center position in the sheet width direction in thecross section parallel to the rolling direction, an analysis isperformed in at least 5 visual fields of a region of 50 μm×50 μm at amagnification of 1000 times and an average value of the lengths of thegrain boundary having a crystal orientation difference of 7° about the<110> direction in the bainite is calculated to obtain L₇. Similarly, anaverage value of the lengths of the grain boundary having a crystalorientation difference of 68° about the <110> direction in the bainiteis calculated to obtain L₆₈. As described above, the orientationdifference of ±4° is allowed.

By dividing the obtained L₇ and L₆₈ by the measurement area, the totaldensity of the length L₇ of the grain boundary having the crystalorientation difference of 7° and the length L₆₈ of the grain boundaryhaving the crystal orientation difference of 68° about the <110>direction in the bainite is obtained. In order to extract only thebainite and measure the density of the length of a specific grainboundary, a region exceeding Iα/2 may be extracted as the bainite, as inthe case of determining the area ratio of the bainite.

Average grain size of prior austenite grains: 10 to 30 μm

Ratio I_(d)/S_(d) between Long Axis I_(d) and Short Axis S_(d) of PriorAustenite Grains: 2.0 or less

In the hot-rolled steel sheet according to the present embodiment, theaverage grain size of the prior austenite grains is 10 to 30 μm, and theratio I_(d)/S_(d) between the long axis I_(d) and the short axis S_(d)of the prior austenite grains may be 2.0 or less. By controlling theaverage grain size of the prior austenite grains and the I_(d)/S_(d)within the above range, the punching, properties of the hot-rolled steelsheet can be improved.

A method of measuring the average grain size of the prior austenitegrains and the ratio I_(d)/S_(d) between the long axis I_(d) and theshort axis S_(d) of the prior austenite grains will be described below.

A test piece is taken from the hot-rolled steel sheet so that amicrostructure at a depth of ¼ of the sheet thickness from the surfaceand a center position in a sheet width direction in the cross sectionparallel to the rolling direction can be observed. The prior austenitegrain boundary is exposed by corroding the observed section with asaturated aqueous solution of picric acid. A magnified photograph of across section parallel to the rolling direction that has been corroded,at a depth of ¼ of the sheet thickness from the surface and at thecenter position, in the sheet width direction is photographed with ascanning electron microscope (SEM) at a magnification of 1000 times and5 or more visual fields. The equivalent circle diameters (diameters) ofat least 20 prior austenite grains having an equivalent circle diameter(diameter) of 2 μm or more, which are included in each SEM photograph,are determined by image processing, and an average value thereof iscalculated to obtain the average grain size of the prior austenitegrains. In a case where the prior austenite grains having an equivalentcircle diameter of less than 2 μm are included, the above measurement isperformed by excluding these grains.

In addition, the long axis and the short axis of at least 20 prioraustenite grains having an equivalent circle diameter (diameter) of 2 μmor more, which are included in each of the above SEM photographs, aremeasured. By calculating the average value of the long axis and theshort axis obtained by measuring each prior austenite grain, the longaxis id and the short axis S_(d) of the prior austenite grain areobtained. By calculating these ratios, the ratio I_(d)/S_(d) between thelong axis I_(d) and the short axis S_(d) of the prior austenite rains isobtained.

Tensile Strength: 780 MPa or More

The-hot-rolled steel sheet according to the present embodiment has atensile (maximum) strength of 780 MPa or more. When the tensile strengthis less than 780 MPa, an applicable component is limited, and thecontribution of weight reduction of the vehicle body is small. Thetensile strength is preferably 980 MPa or more. An upper limit is notparticularly limited, and may be 1800 MPa from the viewpoint ofsuppressing wearing of a die.

Total Elongation: 14.0% or More

The hot-rolled steel sheet according to the present embodiment may havea total elongation of 14.0% or more. An upper limit of the totalelongation is not particularly limited, and may be 30.0% or less or25.0% or less.

The tensile strength and the total elongation are measured according toJIS Z 2241: 2011 using a No. 5 test piece of JIS Z 2241: 2011. Asampling position of the tensile test piece is set to the centerposition in the sheet width direction, and the direction perpendicularto the rolling direction may be the longitudinal direction. Thecross-head speed is set to 3 mm/min.

Hole Expansion Rate: 50% or More

The hot-rolled steel sheet according to the present embodiment may havea hole expansion rate of 50% or more. It is not necessary toparticularly limit an upper limit of the hole expansion rate, and theupper limit thereof may be 90% or less or 85% or less.

The hole expansion rate is obtained, by performing a hole expanding testin accordance with JIS Z 2256: 2010.

Impact Value at −40° C.: 60 J/cm² or More

The hot-rolled steel sheet according to the present embodiment may havean impact value of 60 or more at −40° C. It is not necessary toparticularly limit an upper limit of the impact value at −40° C., andthe upper limit thereof may be 180 J/cm² or less or 175 J/cm² or less.

A sub-sized Charpy impact test piece is taken from a predeterminedposition of the hot-rolled steel sheet, and the impact value at −40° C.is determined in accordance, with a test method described in JIS Z 2242:2005.

Sheet Thickness: 0.6 to 8.0 mm

The sheet thickness of the hot-rolled steel sheet according to thepresent embodiment is not particularly limited and may be (16 to 8.0 mm.When the sheet thickness of the steel sheet is less than 0.6 mm, itbecomes difficult to secure the rolling completion temperature and therolling force becomes excessive, which may make hot rolling difficult.Therefore, the sheet thickness of the steel sheet according to thepresent embodiment may be set to 0.6 mm or more. The sheet thickness ispreferably 1.2 mm or more or 1.4 mm or more. On the other hand, when thesheet thickness is more than 8.0 mm, it becomes difficult to refine themicrostructure, particularly, the prior austenite grains, and it maybecome difficult to secure the microstructure described above from theviewpoint of the microstructural fraction. Therefore, the sheetthickness may be set to 8.0 mm or less. The sheet thickness ispreferably 6.0 mm or less.

Plating Layer

The-hot-rolled steel sheet according to the present embodiment havingthe above-described chemical composition and microstructure may be asurface-treated steel sheet provided with a plating layer on the surfacefor the purpose of improving corrosion resistance and the like. Theplating layer may be an electro plating layer or a hot-dip platinglayer. Examples of the electro plating layer include electrogalvanizingand electro Zn—Ni alloy plating. Examples of the hot-dip plating layerinclude hot-dip galvanizing, hot-dip galvannealing, hot-dip aluminumplating, hot-dip Zn—Al alloy plating, hot-dip Zn—Al—Mg alloy plating,and hot-dip Zn—Al—Mg—Si alloy plating. The plating adhesion amount isnot particularly limited and may be the same as before. Further, it isalso possible to further enhance the corrosion resistance by applying anappropriate chemical conversion treatment (for example, application anddrying of a silicate-based chromium-free chemical conversion treatmentliquid) after plating.

Next, a preferred method of manufacturing the hot-rolled steel sheetaccording to the present embodiment will be described.

The preferred method of manufacturing the hot-rolled steel sheetaccording to the present embodiment includes the following steps. Thetemperature of the slab and the temperature of the steel sheet in thepresent embodiment refer to the surface temperature of the slab and thesurface temperature of the steel sheet.

A heating step of retaining a slab having a predetermined chemicalcomposition at a heating temperature of 1200° C. or higher for 1.0 houror longer.

A hot rolling step of performing rough rolling so that a rough rollingcompletion temperature is 1000° C. or higher and a total rollingreduction is more than 65%, and performing finish rolling so that afinish rolling completion temperature is 860° C. to 980° C.

A cooling step of performing cooling to a temperature range of 570° C.to 620° C. at an average cooling rate of 20° C./s or higher andperforming winding, then, performing retaining at a temperature range of500° C. to 580° C. for 2.0 to 12.0 hours, and then performing cooling toa room temperature.

In the hot rolling step, a total rolling reduction in the rough rollingis set to 70% or more, and the finish rolling may be performed so thatall rolling reductions of final three stages of the finish rolling areless than 25%.

Each step will be described in detail below.

Heating Step

In the heating step, the slab having the above-mentioned chemicalcomposition is heated to a heating temperature of 1200° C. or higher andretained for 1.0 hour. Since a coarse precipitate present at a slabstage causes cracking during rolling and a decrease in materialproperties, it is preferable to heat a steel material before the hotrolling to dissolve the coarse carbide. Therefore, the heatingtemperature is set to 1200° C. or higher, and the retention time is setto 1.0 hour or longer. The preferred heating temperature is 1230° C. orhigher, and the preferred retention time is 3.0 hours or longer.

On the other hand, when the heating, temperature becomes too high or theretention time becomes too long, the yield may decrease due to the largeamount of scale generated. Therefore, the heating temperature may be setto 1400° C. or lower, and the retention time may be set to 20.0 hours orshorter.

The slab to be heated is preferably produced by continuous casting fromthe viewpoint of manufacturing cost, but may be produced by anothercasting method (for example, ingot-making method).

Hot Rolling Step

When the rough rolling is performed at a temperature lower than 1000°C., the prior austenite grains are not sufficiently recrystallized.Therefore, the texture develops and the desired hole expansibilitycannot be obtained. Therefore, rough rolling is performed so that therough rolling completion temperature is 1000° C. or higher. The roughrolling completion temperature is preferably 1050° C. or higher. On theother hand, when the rough rolling is performed at a temperature higherthan 1300° C., the yield may decrease in some cases due to an increasein the amount of scale generated. Therefore, the rough rollingcompletion temperature may be 1300° C. or lower.

In a case where the total rolling reduction in the rough rolling is low,grain sizes of the prior austenite grains becomes non-uniform, whichcauses a decrease in toughness. Therefore, the total rolling reductionin the rough rolling is set to more than 65%. The total rollingreduction in the rough rolling is preferably 68% or more, morepreferably 70% or more, and even more preferably 80% or more. An upperlimit of the total rolling reduction in the rough rolling is notparticularly limited, and may be set to 90% or less.

The total rolling, reduction in the rough rolling is represented by(1−t_(r)/t_(s))×100 (%) using the slab thickness: t_(s) and the sheetthickness t_(r) at the end of the rough rolling.

By setting the total rolling reduction in the rough rolling to 70% ormore and strictly controlling the rolling reduction in the final threestages of finish rolling as described later, the average grain size andan aspect ratio of the prior austenite grains described above can berealized.

When the finish rolling completion temperature is lower than 860° C.,the prior austenite grains are not sufficiently recrystallized.Therefore, the texture develops and the hole expansibility deteriorates.Therefore, the finish rolling completion temperature is set to 860° C.or higher. The finish rolling completion temperature is preferably setto 900° C. or higher. On the other hand, when the finish rollingcompletion temperature is higher than 980° C., the prior austenitegrains become significantly coarse and the desired toughness cannot beobtained. Therefore, the finish rolling completion temperature is set to980° C. or lower. The finish rolling completion temperature ispreferably 950° C. or lower.

In the present embodiment, in order to realize the above-mentionedaverage grain size and the aspect ratio of the prior austenite grainsand improve the punching properties of the hot-rolled steel sheet, thetotal rolling reduction in the rough rolling and rolling reduction offinal three stages of the finish rolling may be strictly controlled.Specifically, as described above, the total rolling reduction in therough, rolling may be set to 70% or more, and the rolling reduction inthe final three stages of the finish rolling may be set to less than25%.

When even one rolling reduction among the rolling reductions of thefinal three stages of the finish rolling, that is, among the rollingreductions of the final pass of the finish rolling, second pass from thefinal pass, and the third pass from the final pass is 25% or more, theprior austenite grains become flat due to the rolling, and the prioraustenite grains having a large aspect ratio, which is the startingpoint of a crack during punching, are formed. Therefore, all of therolling reductions of the final three stages of the finish rolling (therolling reductions of the final pass of the finish rolling, the secondpass from the final pass, and the third pass from the final pass) may beset to less than 25%. All of the rolling reductions are 20% or less. Therolling reduction can be represented by (1−h/h₀)×100 (%) when the sheetthickness after rolling in one pass is h and the sheet thickness beforerolling is h₀.

Cooling Step

After the hot rolling step, cooling is performed to a temperature rangeof 570° C. to 620° C. at an average cooling rate of 20° C./s or higher.In the present embodiment, the average cooling rate is a value obtainedby dividing the temperature difference between the start point and theend point of the set range by the elapsed time from the start point tothe end point.

When the average cooling rate is lower than 20° C./s, a large amount offerrite precipitates and a desired amount of bainite cannot be obtained.Therefore, the average cooling rate is set to 20° C./s or higher. Theaverage cooling rate is preferably 30° C./s or higher, and morepreferably 50° C./s or higher. From the viewpoint of suppressing theincrease in cooling equipment, the average cooling rate may be 200° C./sor lower.

Cooling at the average cooling rate of 20° C./s or higher is performedto a temperature range of 570° C. to 620° C. When a cooling stoptemperature is higher than 620° C., a desired amount of bainite cannotbe obtained. Therefore, the cooling stop temperature is set to 620° C.or lower. The cooling stop temperature may be any temperature as long asit can be retained in a temperature range of 620° C. or lower and 500°C. to 580° C., and in order to retain in the temperature range of 500°C. to 580° C. for 2.0 hours or more, the cooling stop temperature ispreferably set to 550° C. or higher. In addition, in order to preferablycontrol the total density of L₇ and L₆₈ and obtain excellent toughness,the cooling stop temperature is preferably set to 570° C. or higher.

Although the cooling stop temperature is lower than 500° C. andretention is performed in the temperature range of 500° C. to 580° C. byreheating, a desired amount of bainite cannot be obtained. Therefore,heating after the stop of cooling is not desirable.

After cooling with an average cooling rate of 20° C./s or higher,winding is performed. After winding, retaining is performed in atemperature range of 500° C. to 580° C. for 2.0 to 12.0 hours. When theretention temperature is outside the temperature range of 500° C. to580° C., or when the retention time is shorter than 2.0 hours or longerthan 12.0 hours, a total density of L₇ and L₆₈ in the desired amount ofbainite cannot be obtained. Therefore, the retention temperature is setto a temperature range of 500° C. to 580° C., and the retention time isset to 2.0 to 12.0 hours. A lower limit of the retention temperature ispreferably 530° C. An upper limit of the retention temperature ispreferably 560° C. A lower limit of the retention time is preferably 4.0hours, and more preferably 6.0 hours. An upper limit of the retentiontime is preferably 10.0 hours, and more preferably 8.0 hours.

During retaining in the temperature range of 500° C. to 580° C., thesteel sheet temperature may be fluctuated or be constant in thetemperature range of 500° C. to 580° C. In addition, even when thecooling stop temperature of cooling having an average cooling rate of20° C./s or higher is lower than 580° C., it is sufficient that theretention time of 2.0 to 12.0 hours can be secured in the temperaturerange of 500° C. to 580° C.

After performing the above-mentioned retaining in the temperature rangeof 500° C. to 580° C., cooling is performed to a room temperature. Asthe method of cooling to a room temperature, any method may be used, andcooling may be performed by an appropriate method such as mist coolingor rapid cooling using a water cooling tank, in addition to air cooling.The room temperature referred to here is a temperature range of 20° C.to 30° C.

EXAMPLES

Next, the effects of one aspect of the present invention will bedescribed more specifically by way of examples, but the conditions inthe examples are condition examples adopted for confirming thefeasibility and effects of the present invention. The present inventionis not limited to these condition examples. The present invention canemploy various conditions as long as the object of the present inventionis achieved without departing from the gist of the present invention.

Steels having chemical compositions shown in Steel Nos. A to AM in Table1 were melted and continuously cast to manufacture slabs having athickness of 240 to 300 mm. The obtained slabs were used to obtainhot-rolled steel sheets shown in Tables 5 to 7 under the manufacturingconditions shown in Tables 2 to 4. In, addition, “F1”, “F2”, and “F3” inTables 2 to 4 represent a rolling reduction of the final pass of finishrolling, a rolling reduction of the second pass from the final pass, anda roiling reduction of the third pass from the final pass, respectively.In addition, a test material No. 63 in Table 4 was reheated after thestop of cooling, and then retained in the temperature range of 500° C.to 580° C.

With respect to the obtained hot-rolled steel sheet, the microstructuralfraction, the total density of L₇ and L₆₈, the average grain size of theprior austenite grains, and the ratio I_(d)/S_(d) between the long axisI_(d) and the short, axis S_(d) of the prior austenite grains weredetermined. The results obtained are shown in Tables 5 to 7.

Evaluation Method of Properties of Hot-Rolled Steel Sheet

Tensile Strength (TS) and Total Elongation (EI)

Among the mechanical properties of the obtained hot-rolled steel sheet,the tensile strength (TS) and total elongation (EI) were measured byusing a test piece No 5 of JIS Z 2241: 2011, in accordance with JIS Z2241: 2011. A sampling position of the tensile test piece was set to thecenter position, in the sheet width direction, and the directionperpendicular to the rolling direction was set to the longitudinaldirection. The cross-head speed was set to 3 mm/min.

In a case where the tensile strength (TS) was 780 MPa or more, thestrength was determined excellent which was pass, and in a case wherethe tensile strength was less than 780 MPa, the strength was determinedpoor which was fail. In addition, in a case where the total elongation(EI) was 14.0% or more, the ductility was determined excellent which waspass, and in a case where the total elongation was less than 14.0, theductility was determined poor which was fail.

Hole Expansion Rate (λ)

The hole expansion rate (λ) was evaluated by performing a hole expandingtest in accordance with JIS Z 2256: 2010.

In a case where the hole expansion rate (λ) was 50% or more, the holeexpansibility as determined excellent which was pass, and in a casewhere hole expansion rate (λ) was less than 50%, the hole expansibilitywas determined, poor which was fail.

Impact Value (vE₄₀)

The toughness was evaluated by performing a Charpy impact test at −40°C. and determining the impact value. A sub-sized Charpy impact testpiece was taken from a predetermined position of the hot-rolled steelsheet, and the impact value at −40° C. was determined in accordance witha test method described in JIS Z 2242: 2005 to evaluate the toughness.

In a case where the impact value (vE₄₀) was 60 J/cm² or more, thetoughness was determined excellent which was pass, and in a case wherethe impact value (vE₄₀) was less than 60 J/cm², the toughness wasdetermined poor which was fail.

Punching Properties

The punching properties were evaluated by performing a punching test andobserving the properties of the punched end surface. First, a punchedhole was prepared with a hole diameter of 10 mm, a clearance of 12.5%,and a punching speed of 80 mm/s. Next, a cross section of the punchedhole perpendicular to the direction was embedded in a resin, and, thepunched end surface was imaged with a scanning electron microscope. In acase where the obtained observation photographs were observed and endsurface roughness was not observed, “E (Excellent)” was noted in Tables5 to 7 as having particularly good punching properties. In addition, ina case where a small elopement of less than 100 um was observed, “G(Good)” is noted in Tables 5 to 7 as having good punching properties,and in a case where a large elopement of 100 μm or more is observed “B(Bad)” is noted in Tables 5 to 7 as having poor punching properties.

When referring to Tables 5 to 7, it can be seen that Invention Exampleshave high strength and excellent ductility, hole expansibility, andtoughness. In addition, it can be, seen that Invention Examples in whichthe average grain size of the prior austenite grains is 10 to 30 μm, andthe, ratio I_(d)/S_(d) between the long axis I_(d) and the short axisS_(d) of the prior austenite grains is 2.0 or less have particularlygood punching properties.

On the other hand, it can be seen that Comparative Example is poor inany one or more of the strength, the ductility, the hole expansibilityand toughness.

TABLE 1 Kind of Chemical composition (unit: mass %, remainder consistingof Fe and impurities) steel C Si Mn sol. Al Ti P S N Others Remarks A0.070 0.90 2.20 0.050 0.120 0.010 0.001 0.003 Present Invention Steel B0.210 0.50 2.50 0.030 0.100 0.010 0.001 0.005 Comparative Steel C 0.0281.20 2.20 0.050 0.090 0.010 0.002 0.003 Comparative Steel D 0.090 2.802.20 0.035 0.110 0.010 0.001 0.003 Comparative Steel E 0.063 1.40 0.850.100 0.110 0.020 0.001 0.003 Comparative Steel F 0.070 1.00 4.50 0.0300.060 0.010 0.002 0.003 Comparative Steel G 0.041 1.50 1.50 2.200 0.1100.020 0.001 0.002 Comparative Steel H 0.040 1.01 1.50 0.030 0.020 0.0100.001 0.003 Comparative Steel I 0.102 1.20 1.90 0.030 0.250 0.010 0.0010.003 Comparative Steel J 0.178 0.55 1.65 0.090 0.110 0.009 0.002 0.003Present Invention Steel K 0.033 1.67 2.43 0.030 0.130 0.010 0.001 0.003Present Invention Steel L 0.055 1.55 1.85 0.040 0.100 0.010 0.002 0.003Nb: 0.020 Present Invention Steel M 0.089 1.25 2.00 0.100 0.090 0.0100.002 0.003 Nb: 0.230 Comparative Steel N 0.065 1.20 2.10 0.020 0.1100.005 0.001 0.002 Nb: 0.012, Cr: 0.50 Present Invention Steel O 0.0601.13 2.41 0.025 0.112 0.010 0.001 0.002 Nb: 0.020, B: 0.002 PresentInvention Steel P 0.049 1.20 1.80 0.060 0.090 0.010 0.003 0.003 B: 0.001Present Invention Steel Q 0.057 0.90 1.60 0.020 0.110 0.009 0.003 0.003Cr: 0.70, B: 0.002 Present Invention Steel R 0.060 0.53 1.95 0.050 0.1500.010 0.002 0.002 V: 0.01 Present Invention Steel S 0.101 0.30 1.670.020 0.120 0.007 0.001 0.003 Mo: 0.02 Present Invention Steel T 0.0701.20 1.50 0.030 0.100 0.010 0.002 0.003 Mo: 0.03, V: 0.03 PresentInvention Steel U 0.062 1.10 2.20 0.013 0.080 0.010 0.002 0.001 Cr:0.61, Mo: 0.02, Present Invention Steel V: 0.10 V 0.090 0.33 1.60 0.1200.110 0.010 0.001 0.002 Cr: 0.45, Mo: 0.25, Present Invention Steel V:0.18, Ca: 0.003 W 0.081 0.63 1.55 0.130 0.099 0.011 0.003 0.003 Cr:0.61, Mo: 0.20, Present Invention Steel V: 0.24, B: 0.002 X 0.055 0.952.50 0.122 0.110 0.009 0.003 0.002 Mo: 1.30 Comparative Steel Y 0.0710.10 2.80 0.500 0.130 0.008 0.002 0.003 Cu: 0.05 Present Invention SteelZ 0.053 1.00 1.50 0.050 0.130 0.002 0.003 0.003 Ni: 0.80 PresentInvention Steel AA 0.068 1.30 2.00 0.030 0.060 0.002 0.002 0.003Ni: 1.30 Comparative Steel AB 0.100 1.12 1.66 0.020 0.110 0.003 0.0020.003 Co: 0.51 Present Invention Steel AC 0.055 1.03 1.78 0.030 0.1000.004 0.002 0.003 Ca: 0.040 Present Invention Steel AD 0.081 0.90 2.030.030 0.120 0.010 0.001 0.003 Mg: 0.008 Present Invention Steel AE 0.0561.50 1.91 0.022 0.070 0.010 0.001 0.003 REM: 0.005 Present InventionSteel AF 0.068 1.87 1.59 0.050 0.090 0.009 0.001 0.003 Zr: 0.003 PresentInvention Steel AG 0.071 1.00 1.72 0.030 0.110 0.008 0.001 0.002 Cr:0.64 Present Invention Steel AH 0.070 0.98 2.01 0.031 0.150 0.015 0.0020.003 Nb: 0.050 Present Invention Steel AI 0.180 1.71 2.58 0.016 0.0910.015 0.002 0.003 B: 0.001 Present Invention Steel AJ 0.071 1.20 2.100.040 0.090 0.007 0.002 0.003 W: 0.050 Present Invention Steel AK 0.0800.08 2.20 0.020 0.100 0.003 0.002 0.003 Present Invention Steel AL 0.0652.20 2.00 0.001 0.080 0.002 0.001 0.003 Present Invention Steel AM 0.0721.40 1.20 0.070 0.130 0.002 0.001 0.003 Present Invention Steel Anunderline indicates that the value is outside a range of the presentinvention.

TABLE 2 Manufacture conditions Heating step Rough rolling step Finishrolling step Sample Heating Retention Rolling completion Total rollingRolling reduction Rolling start Rolling completion material Kind oftemperature time temperature reduction (%) temperature temperature No.steel (° C.) (h) (° C.) (%) F1 F2 F3 (° C.) (° C.) 1 N 1244 2.3 1146 8228 25 9 1060 890 2 A 1250 3.0 1156 81 26 26 11 1055 899 3 U 1263 3.01145 81 26 26 11 1054 901 4 A 1255 2.6 1152 80 25 25 14 1063 879 5 L1222 3.0 1159 83 25 26 13 1051 878 6 B 1254 3.1 1154 82 28 18 12 1055904 7 C 1260 3.2 1163 81 30 13 12 1066 900 8 D 1222 3.5 1133 82 28 15 91079 878 9 E 1248 2.8 1121 80 20 25 25 1025 881 10 F 1253 2.5 1145 83 2815 9 1034 894 11 G 1255 3.3 1146 82 30 11 9 1028 900 12 H 1247 2.5 114381 27 22 10 1053 892 13 I 1231 4.5 1090 80 28 20 10 1045 888 14 J 12443.1 1162 81 26 26 11 1049 901 15 K 1261 3.6 1141 84 26 18 10 1054 887 16L 1268 3.5 1151 82 28 20 10 1061 893 17 M 1255 3.1 1100 85 26 20 8 1045906 18 N 1262 2.1 1133 78 26 20 12 1080 910 19 O 1251 3.0 1143 80 30 119 1064 897 20 P 1231 1.7 1131 76 26 18 10 1045 899 21 Q 1222 1.5 1108 8127 18 8 1056 901 22 R 1258 2.3 1121 80 30 13 12 1054 911 23 S 1255 3.21151 79 28 18 10 1056 904 24 T 1252 2.2 1109 80 26 26 11 1033 874 25 U1266 3.1 1143 80 26 22 12 1043 889 Manufacture conditions Cooling stepAverage cooling Sample rate after Cooling stop Retention start Retentionend Retention Cooling material finish rolling temperature temperaturetemperature time method after No. (° C./s) (° C.) (° C.) (° C.) (h)winding Remarks 1 100 592 580 500 1.7 Air cooling Comparative Example 280 596 580 500 3.1 Air cooling Invention Example 3 87 589 580 500 5.3Air cooling Invention Example 4 100 596 580 500 11.1  Air coolingInvention Example 5 100 599 580 500 14.2  Air cooling ComparativeExample 6 122 582 580 500 3.5 Air cooling Comparative Example 7 125 592580 500 4.3 Air cooling Comparative Example 8 100 584 580 500 4.2 Aircooling Comparative Example 9 45 593 580 500 2.2 Air cooling ComparativeExample 10 41 581 580 500 8.0 Air cooling Comparative Example 11 67 595580 500 4.6 Air cooling Comparative Example 12 98 583 580 500 2.6 Aircooling Comparative Example 13 80 585 580 500 3.5 Air coolingComparative Example 14 85 589 580 500 4.1 Air cooling Invention Example15 151 581 580 500 5.2 Air cooling Invention Example 16 120 594 580 5003.4 Air cooling Invention Example 17 150 586 580 500 6.2 Air coolingComparative Example 18 120 589 580 500 4.0 Air cooling Invention Example19 130 588 580 500 7.1 Air cooling Invention Example 20 75 583 580 5002.6 Air cooling Invention Example 21 89 598 580 500 4.5 Air coolingInvention Example 22 110 595 580 500 6.5 Air cooling Invention Example23 120 593 580 500 4.7 Air cooling Invention Example 24 44 590 580 5007.4 Air cooling Invention Example 25 130 591 580 500 5.1 Air coolingInvention Example An underline indicates that the value is outside arange of the present invention.

TABLE 3 Manufacture conditions Heating step Rough rolling step Finishrolling step Sample Heating Retention Rolling completion Total rollingRolling reduction Rolling start Rolling completion material Kind oftemperature time temperature reduction (%) temperature temperature No.steel (° C.) (h) (° C.) (%) F1 F2 F3 (° C.) (° C.) 26 V 1265 2.8 1154 7826 18 10 1055 901 27 W 1255 3.1 1143 81 28 18 15 1066 920 28 X 1241 1.61151 80 26 15 12 1065 895 29 Y 1255 2.5 1163 82 28 15 10 1054 901 30 Z1266 1.8 1128 77 26 17 12 1061 899 31 AA 1263 2.3 1139 80 26 18 10 1045879 32 AB 1231 1.9 1154 74 30 11 9 1053 931 33 AC 1261 3.7 1163 82 28 1210 1045 904 34 AD 1255 3.2 1155 81 30 11 9 1051 879 35 AE 1224 3.1 114873 26 18 10 1059 911 36 AF 1245 2.3 1147 81 28 14 12 1061 893 37 AG 12481.7 1161 82 30 22 11 1056 897 38 A 1238 3.1 1158 81 22 22 18 1061 920 39L 1255 2.1 1167 81 18 18 15 1053 910 40 P 1245 3.4 1151 82 20 20 22 1054903 41 Q 1248 3.1 1133 81 19 19 12 1071 921 42 S 1247 2.7 1181 82 15 1210 1061 910 43 T 1243 1.6 1136 81 18 18 12 1056 899 44 U 1251 2.6 115280 20 18 10 1051 900 45 V 1255 2.5 1148 83 22 16 12 1065 911 46 W 12603.5 1139 82 21 16 12 1055 915 47 AC 1248 3.1 1157 84 18 18 14 1042 90948 AD 1251 3.0 1141 81 22 15 9 1050 912 49 R 1255 3.1 1152 80 20 15 111060 906 50 AG 1248 2.9 1144 82 18 18 17 1061 913 Manufacture conditionsCooling step Average cooling Sample rate after Cooling stop Retentionstart Retention end Retention Cooling material finish rollingtemperature temperature temperature time method after No. (° C./s) (°C.) (° C.) (° C.) (h) winding Remarks 26 100 591 580 500 4.3 Air coolingInvention Example 27 150 585 580 500 2.8 Air cooling Invention Example28 120 598 580 500 3.4 Air cooling Comparative Example 29 85 588 580 5008.5 Air cooling Invention Example 30 95 593 580 500 4.2 Air coolingInvention Example 31 105 595 580 500 5.1 Air cooling Comparative Example32 65 588 580 500 6.2 Air cooling Invention Example 33 80 588 580 5004.6 Air cooling Invention Example 34 180 582 580 500 3.9 Air coolingInvention Example 35 49 595 580 500 9.8 Air cooling Invention Example 36120 588 580 500 7.3 Air cooling Invention Example 37 80 598 580 500 3.2Air cooling Invention Example 38 141 588 580 500 5.5 Air coolingInvention Example 39 108 587 580 500 3.1 Air cooling Invention Example40 72 590 580 500 2.9 Air cooling Invention Example 41 54 588 580 5005.2 Air cooling Invention Example 42 100 591 580 500 3.9 Air coolingInvention Example 43 70 593 580 500 4.5 Air cooling Invention Example 44150 595 580 500 2.9 Air cooling Invention Example 45 120 589 580 500 3.5Air cooling Invention Example 46 110 593 580 500 3.1 Air coolingInvention Example 47 100 587 580 500 5.1 Air cooling Invention Example48 150 598 580 500 7.9 Air cooling Invention Example 49 112 591 580 5004.5 Air cooling Invention Example 50 121 589 580 500 2.9 Air coolingInvention Example An underline indicates that the value is outside arange of the present invention.

TABLE 4 Manufacture conditions Heating step Rough rolling step Finishrolling step Sample Heating Retention Rolling completion Total rollingRolling reduction Rolling start Rolling completion material Kind oftemperature time temperature reduction (%) temperature temperature No.steel (° C.) (h) (° C.) (%) F1 F2 F3 (° C.) (° C.) 51 Y 1251 3.5 1147 8024 20 18 1065 911 52 Z 1241 2.5 1161 74 20 20 20 1053 889 53 AB 1245 2.51138 79 22 18 20 1061 908 54 AE 1235 4.2 1157 81 20 20 16 1058 893 55 AF1246 3.1 1148 77 21 20 18 1067 908 56 A 1138 1.5 1085 81 30 18 15 1023899 58 A 1261 2.5 1153 81 31 16 10 1033 840 59 J 1256 2.7 1152 81 30 2015 1091 995 60 K 1283 3.1 1148 80 28 21 13 1064 905 61 K 1260 2.8 116282 28 20 14 1071 894 62 K 1253 3.1 1183 81 30 22 11 1055 903 63 K 12652.5 1165 80 28 19 13 1068 901 64 J 1241 2.8 1156 85 30 17 12 1067 911 65J 1263 2.2 1147 84 28 25 12 1076 899 66 AH 1259 1.8 1151 76 40 40 401058 910 67 AH 1261 2.1 1139 81 22 20 15 1056 915 68 AI 1251 3.5 1145 7655 50 45 1095 950 69 AI 1261 4.0 1153 68 24 24 18 1065 901 70 AJ 12583.2 1161 81 22 20 19 1081 892 71 AJ 1249 2.9 1150 76 28 26 20 1045 88372 A 1250 3.5 1154 62 30 18 14 1051 900 73 AH 1249 3.1 1152 83 30 22 111042 903 74 AK 1249 2.6 1161 72 24 15 13 1048 890 75 AL 1260 1.8 1158 7530 18 15 1035 895 76 AM 1250 2.4 1155 74 28 20 16 1052 915 Manufactureconditions Cooling step Average cooling Sample rate after Cooling stopRetention start Retention end Retention Cooling material finish rollingtemperature temperature temperature time method after No. (° C./s) (°C.) (° C.) (° C.) (h) winding Remarks 51 102 591 580 500 6.1 Air coolingInvention Example 52 100 596 580 500 3.5 Air cooling Invention Example53 120 587 580 500 4.6 Air cooling Invention Example 54  80 590 580 5004.8 Air cooling Invention Example 55 101 587 580 500 6.2 Air coolingInvention Example 56 130 586 580 500 5.1 Air cooling Comparative Example58 130 580 580 500 3.3 Air cooling Comparative Example 59  89 598 580500 4.2 Air cooling Comparative Example 60  15 595 580 500 6.0 Aircooling Comparative Example 61  35 596 580 500 3.7 Air cooling InventionExample 62  80 630 580 500 7.2 Air cooling Comparative Example 63  78 381* 568 500 3.5 Air cooling Comparative Example 64 120 573 573 500 3.7Air cooling Invention Example 65 100 577 577 513 2.3 Water coolingInvention Example 66  80 520 520 500 0.7 Air cooling Comparative Example67 150 589 580 500 4.1 Air cooling Invention Example 68 120 550 550 5001.4 Air cooling Comparative Example 69 130 581 580 500 5.6 Air coolingInvention Example 70 150 589 580 500 6.8 Air cooling Invention Example71 120 579 579 500 4.2 Air cooling Invention Example 72 105 583 580 5003.5 Air cooling Comparative Example 73 110 560 560 500 2.1 Air coolingComparative Example 74  99 584 580 500 3.1 Air cooling Invention Example75 105 591 580 500 3.5 Air cooling Invention Example 76 108 578 578 5002.8 Air cooling Invention Example An underline indicates that the valueis outside a range of the present invention. *Reheating after stop ofcooling

TABLE 5 Microstructure “Martensite + Average tempered martensite” grainsize Sample Remainder in in remainder in of prior l_(d)/S_(d) materialKind of Bainite Ferrite microstructure microstructure L₇ + L₆₈ γ grainsof prior No. steel (area %) (area %) (area %) (area %) (μm/μm²) (μm) γgrains  1 N 83.2 3.1 13.7  9.0 0.64 18 3.1  2 A 92.5 4.0 3.5 2.0 0.57 262.7  3 U 91.3 6.5 2.2 1.0 0.51 29 2.8  4 A 94.1 3.2 2.7 2.5 0.41 20 3.0 5 L 87.4 9.3 3.3 2.6 0.30 24 3.4  6 B 51.2 0.4 48.4  45.2 0.55 22 3.4 7 C 35.0 45.0  20.0  3.0 0.41 18 3.1  8 D 76.3 1.2 22.5  11.0 0.51 153.2  9 E 76.2 19.3  4.5 1.2 0.52 18 3.5 10 F 31.0 0.2 68.8  61.0 0.42 142.5 11 G 61.3 32.1  6.6 3.2 0.44 18 2.2 12 H 85.6 9.1 5.3 4.1 0.56 242.4 13 I 86.3 9.8 3.9 3.2 0.57 14 4.1 14 J 88.9 1.6 9.5 8.6 0.51 20 2.315 K 92.1 4.5 3.4 2.1 0.47 21 2.4 16 L 91.2 3.1 5.7 5.2 0.53 17 2.8 17 M91.3 6.3 2.4 2.1 0.43 18 3.6 18 N 93.5 2.6 3.9 3.5 0.55 23 2.1 19 O 91.41.5 7.1 6.3 0.47 21 2.2 20 P 86.1 8.3 5.6 4.2 0.54 20 2.3 21 Q 94.5 1.54.0 2.1 0.51 22 2.1 22 R 87.3 8.2 4.5 3.1 0.47 20 2.7 23 S 91.2 1.2 7.66.7 0.53 16 2.8 24 T 91.0 4.3 4.7 4.2 0.45 17 2.6 25 U 90.5 2.1 7.4 5.10.54 19 2.5 Mechanical properties Punching property Sample StrengthWorkability Toughness Property of material TS El λ vE₄₀ punched end No.(MPa) (%) (%) (J/cm²) surface Remarks  1 1046  12.6 44 103  GComparative Example  2 999 14.6 70 89 G Invention Example  3 990 16.0 7368 G Invention Example  4 983 17.0 74 67 G Invention Example  5 901 19.072 35 B Comparative Example  6 1251  10.0 35 65 G Comparative Example  7701 23.0 72 31 B Comparative Example  8 1065  17.2 24 44 G ComparativeExample  9 756 21.3 65 41 B Comparative Example 10 1089  10.2 48 81 GComparative Example 11 776 23.0 62 51 G Comparative Example 12 738 25.071 40 G Comparative Example 13 981 14.2 31 52 B Comparative Example 14996 15.2 61 121  G Invention Example 15 803 20.8 72 78 G InventionExample 16 993 15.2 61 89 G Invention Example 17 1001  14.3 28 71 GComparative Example 18 1021  15.4 66 108  G Invention Example 19 98316.2 60 98 G Invention Example 20 956 15.1 67 83 G Invention Example 211035  16.2 67 72 G Invention Example 22 1021  16.3 71 99 G InventionExample 23 1098  14.8 54 102  G Invention Example 24 1033  15.3 66 82 GInvention Example 25 1100  15.4 61 120  G Invention Example An underlineindicates that the value is outside a range of the present invention orthat the property is not preferable.

TABLE 6 Microstructure “Martensite + Average tempered martensite” grainsize Sample Remainder in in remainder in of prior l_(d)/S_(d) materialKind of Bainite Ferrite microstructure microstructure L₇ + L₆₈ γ grainsof prior No. steel (area %) (area %) (area %) (area %) (μm/μm²) (μm) γgrains 26 V 89.4 4.3 6.3 5.2 0.51 20 2.1 27 W 92.8 2.1 5.1 4.6 0.56 242.3 28 X 73.2 2.4 24.4  21.5 0.56 12 3.8 29 Y 83.1 8.1 8.8 6.2 0.38 192.3 30 Z 88.2 3.5 8.3 7.1 0.54 21 2.2 31 AA 72.5 0.6 26.9  24.1 0.49 172.7 32 AB 93.1 1.3 5.6 4.5 0.44 24 2.1 33 AC 88.4 4.1 7.5 5.6 0.51 222.4 34 AD 89.3 3.0 7.7 4.2 0.55 16 2.7 35 AE 90.5 6.1 3.4 3.1 0.48 212.2 36 AF 93.2 2.1 4.7 4.5 0.41 17 2.4 37 AG 92.5 4.2 3.3 3.1 0.52 192.3 38 A 88.3 2.6 9.1 6.3 0.54 25 1.8 39 L 93.2 2.4 4.4 4.1 0.57 25 1.640 P 88.5 5.3 6.2 4.2 0.58 23 1.5 41 Q 92.3 5.2 2.5 1.5 0.47 24 1.7 42 S90.2 1.8 8.0 7.1 0.55 21 1.6 43 T 85.6 5.1 9.3 6.1 0.48 22 1.7 44 U 90.41.8 7.8 6.3 0.57 23 1.4 45 V 92.1 3.1 4.8 4.1 0.54 22 1.8 46 W 92.4 1.16.5 4.8 0.54 25 1.4 47 AC 88.3 6.2 5.5 3.9 0.49 24 1.5 48 AD 86.7 8.15.2 3.2 0.41 22 1.7 49 R 90.2 6.2 3.6 2.6 0.50 21 1.6 50 AG 93.1 2.1 4.83.1 0.56 24 1.7 Mechanical properties Punching property Sample StrengthWorkability Toughness Property of material TS El λ vE₄₀ punched end No.(MPa) (%) (%) (J/cm²) surface Remarks 26 1035 15.8 54 151 G InventionExample 27 1065 15.1 67 115 G Invention Example 28 1108 12.3 44 65 BComparative Example 29 1021 16.3 71 68 G Invention Example 30 1002 14.855 102 G Invention Example 31 1103 11.5 42 151 G Comparative Example 321098 15.1 66 95 G Invention Example 33 999 14.5 64 140 G InventionExample 34 1054 16.9 68 100 G Invention Example 35 1011 16.2 83 110 GInvention Example 36 1045 15.8 66 100 G Invention Example 37 993 15.1 7181 G Invention Example 38 998 16.2 63 70 E Invention Example 39 981 16.372 85 E Invention Example 40 941 17.3 65 78 E Invention Example 41 97516.7 79 81 E Invention Example 42 999 15.3 62 94 E Invention Example 431003 15.8 64 75 E Invention Example 44 1081 16.1 59 100 E InventionExample 45 1011 14.9 68 120 E Invention Example 46 996 16.8 75 98 EInvention Example 47 984 15.2 75 100 E Invention Example 48 989 16.7 67105 E Invention Example 49 1007 15.6 75 100 E Invention Example 50 100114.9 71 90 E Invention Example An underline indicates that the value isoutside a range of the present invention or that the property is notpreferable.

TABLE 7 Microstructure “Martensite + Average tempered martensite” grainsize Sample Remainder in in remainder in of prior l_(d)/S_(d) materialKind of Bainite Ferrite microstructure microstructure L₇ + L₆₈ γ grainsof prior No. steel (area %) (area %) (area %) (area %) (μm/μm²) (μm) γgrains 51 Y 84.1 6.4 9.5 4.5 0.42 25 1.7 52 Z 84.6 6.1 9.3 7.5 0.55 241.6 53 AB 91.2 3.5 5.3 2.8 0.48 25 1.6 54 AE 90.2 4.2 5.6 5.1 0.55 251.7 55 AF 89.2 5.4 5.4 3.2 0.39 24 1.4 56 A 83.7 5.1 11.2  2.5 0.46 162.5 58 A 78.1 12.5  9.4 7.5 0.55 25 3.8 59 J 78.2 1.3 20.5  12.3 0.50 332.5 60 K 76.2 15.9  7.9 5.9 0.38 22 2.1 61 K 84.2 7.8 8.0 4.5 0.50 202.2 62 K 52.0 41.0  7.0 5.6 0.38 22 2.1 63 K 65.4 1.3 33.3  32.1 0.55 212.3 64 J 93.1 1.5 5.4 4.5 0.56 19 2.4 65 J 89.2 1.2 9.6 8.3 0.59 20 2.366 AH 85.1 8.1 6.8 4.0 0.71 21 2.6 67 AH 91.0 5.1 3.9 3.2 0.51 23 1.5 68AI 77.5 14.1  8.4 6.2 0.63 22 3.2 69 AI 91.1 6.3 2.6 2.1 0.45 29 2.8 70AJ 92.2 6.1 1.7 1.4 0.41 26 1.8 71 AJ 88.2 9.4 2.4 1.5 0.54 24 2.6 72 A87.2 6.1 6.7 5.2 0.28 26 3.0 73 AH 91.0 2.1 6.9 2.8 0.25 26 2.6 74 AK85.3 6.3 8.4 4.2 0.42 28 1.8 75 AL 86.2 6.1 7.7 3.4 0.43 25 2.6 76 AM84.5 9.1 6.4 2.6 0.38 27 2.4 Mechanical properties Punching propertySample Strength Workability Toughness Property of material TS El λ vE₄₀punched end No. (MPa) (%) (%) (J/cm²) surface Remarks 51  996 15.3 75 80E Invention Example 52  996 14.9 62 88 E Invention Example 53 1055 16.271 86 E Invention Example 54 1106 15.9 64 105  E Invention Example 551061 16.2 75 93 E Invention Example 56  771 23.0 45 70 G ComparativeExample 58 1023 14.2 35 75 B Comparative Example 59 1013 14.5 56 35 BComparative Example 60  763 22.5 80 75 G Comparative Example 61  79123.0 77 65 G Invention Example 62  705 25.0 90 64 G Comparative Example63 1105 10.5 50 120  G Comparative Example 64 1101 14.5 68 132  GInvention Example 65 1188 14.2 58 165  G Invention Example 66  986 18.045 132  G Comparative Example 67  902 21.0 71 95 E Invention Example 68 956 15.2 48 85 B Comparative Example 69 1021 15.3 71 82 G InventionExample 70 1028 16.3 79 76 E Invention Example 71  997 16.8 66 66 GInvention Example 72 1015 14.1 55 45 B Comparative Example 73 1084 14.651 52 G Comparative Example 74 1045 15.3 62 105  E Invention Example 751162 17.1 54 67 G Invention Example 76  998 16.7 62 89 G InventionExample An underline indicates that the value is outside a range of thepresent invention or that the property is not preferable.

INDUSTRIAL APPLICABILITY

According to the present invention, it is possible to provide ahot-rolled steel sheet having high strength, and excellent ductility,hole expansibility, and toughness, and a method of manufacturing thesame. According to the above preferred aspect according to the presentinvention, it is possible to provide a hot-rolled steel sheet havingexcellent punching properties in addition to the above-mentionedproperties and a method of manufacturing the same.

1. A hot-rolled steel sheet comprising, as a chemical composition, bymass %: C: 0.030% to 0.200%; Si: 0.05% to 2.50%; Mn: 1.00% to 4.00%;sol. Al: 0.001% to 2.000%; Ti: 0.030% to 0.200%, P: 0.020% or less; S:0.020% or less; N: 0.010% or less; Nb: 0% to 0.200%; B: 0% to 0.010%; V:0% to 1.00%; Mo: 0% to 1.00%; Cu: 0% to 1.00%; W: 0% to 1.00%; Cr: 0% to1.00%; Ni: 0% to 1.00%; Co: 0% to 1.00%; Ca: 0% to 0.010%; Mg: 0% to0.010%; REM: 0% to 0.010%; Zr: 0% to 0.010%; and a remainder consistingof iron and impurities, wherein a microstructure contains, by area %,bainite: 80.0% or more, ferrite: 10.0% or less, and a remainder in themicrostructure: 10.0% or less, a total density of a length L₇ of a grainboundary having a crystal orientation difference of 7° and a length L₆₈of a grain boundary having a crystal orientation difference of 68° abouta <110> direction in the bainite is 0.35 to 0.60 μm/μm², and a tensilestrength is 780 MPa or more.
 2. The hot-rolled steel sheet according toclaim 1, wherein the hot-rolled steel sheet includes, as a chemicalcomposition, by mass %, one or more selected from the group of: Nb:0.005% to 0.200%; B: 0.001% to 0.010%; V: 0.005% to 1.00%; Mo: 0.005% to1.00%; Cu: 0.005% to 1.00%; W: 0.005% to 1.00%; Cr: 0.005% to 1.00%; Ni:0.005% to 1.00%; Co: 0.005% to 1.00%; Ca: 0.0005% to 0.010%; Mg: 0.0005%to 0.010%; REM: 0.0005% to 0.010%; and Zr: 0.0005% to 0.010%.
 3. Thehot-rolled steel sheet according to claim 1, wherein in themicrostructure, an average grain size of prior austenite grains is 10 to30 and a ratio I_(d)/S_(d) between a long axis I_(d) and a short axisS_(d) of the prior austenite grains is 2.0 or less.
 4. A method ofmanufacturing the hot-rolled steel sheet according to claim 1,comprising: a heating step of retaining a slab having the chemicalcomposition according to claim 1, at a heating temperature of 1200° C.or higher for 1.0 hour or longer; a hot rolling step of performing roughrolling so that a rough rolling completion temperature is 1000° C. orhigher and a total rolling reduction is more than 65%, and performingfinish rolling so that a finish rolling completion temperature is 860°C. to 980° C.; and a cooling step of performing cooling to a temperaturerange of 570° C. to 620° C. at an average cooling rate of 20° C./s orhigher and performing winding, then, performing retaining at atemperature range of 500° C. to 580° C. for 2.0 to 12.0 hours, and thenperforming cooling to a room temperature.
 5. The method of manufacturingthe hot-rolled steel sheet according to claim 4, wherein in the hotrolling step, the total rolling reduction in the rough rolling is set to70% or more, and the finish rolling is performed so that all rollingreductions of final three stages of the finish rolling are less than25%.
 6. The hot-rolled steel sheet according to claim 2, wherein in themicrostructure, an average grain size of prior austenite grains is 10 to30 μm, and a ratio I_(d)/S_(d) between a long axis I_(d) and a shortaxis S_(d) of the prior austenite grains is 2.0 or less.
 7. A hot-rolledsteel sheet comprising, as a chemical composition, by mass %: C: 0.030%to 0.200%; Si: 0.05% to 2.50%; Mn: 1.00% to 4.00%; sol. Al: 0.001% to2.000%; Ti: 0.030% to 0.200%, P: 0.020% or less; S: 0.020% or less; N:0.010% or less; Nb: 0% to 0.200%; B: 0% to 0.010%; V: 0% to 1.00%; Mo:0% to 1.00%; Cu: 0% to 1.00%; W: 0% to 1.00%; Cr: 0% to 1.00%; Ni: 0% to1.00%; Co: 0% to 1.00%; Ca: 0% to 0.010%; Mg: 0% to 0.010%; REM: 0% to0.010%; Zr: 0% to 0.010%; and a remainder comprising iron andimpurities, wherein a microstructure contains, by area %, bainite: 80.0%or more, ferrite: 10.0% or less, and a remainder in the microstructure:10.0% or less, a total density of a length L₇ of a grain boundary havinga crystal orientation difference of 7° and a length L₆₈ of a grainboundary having a crystal orientation difference of 68° about a <110>direction in the bainite is 0.35 to 0.60 μm/μm², and a tensile strengthis 780 MPa or more.